The present invention relates to RF power amplifiers and, more particularly, to such amplifiers including means for minimizing distortion caused by noise including that resulting from variations in the DC power supply as well as from noise generated by transistor switches located within the RF power amplifiers.
Circuits are known for minimizing signal distortion in RF power amplifiers used in AM transmitters. For example, the U.S. Pat. Nos. to H. Swanson et al. 4,737,731, H. I. Swanson 4,580,111 and D. H. Covill 4,605,910 provide examples of such circuits. Thus, each of these patents discloses circuitry for minimizing modulation distortion of an amplitude modulated RF carrier signal resulting from variations in the DC power supply. In each case, a feed forward technique is employed in which a sample of the input DC voltage signal is obtained and is combined with the input audio signal to compensate for variations in the magnitude of the DC supply voltage prior to supplying the signal to the amplification stages of the transmitter. This feed forward technique, therefore, does not compensate for additional noise resulting as from noisy switches in the amplifier stages.
The H. I. Swanson U.S. Pat. No. 4,580,111 additionally discloses an amplitude modulator which generates an amplitude modulated carrier signal by selectively turning on and off a plurality of RF amplifiers in a digital manner to produce amplitude modulation. Each of the RF amplifiers includes a plurality of switching transistors, each of which takes the form of a MOSFET transistor, connected together in a bridge circuit. The output terminals of each bridge circuit are connected across a primary winding of a transformer having a secondary winding. The secondary windings of all of the power amplifiers are connected together in series for combining the amplified RF signals. The combined RF signal is supplied to a load, such as an antenna. The output of the power amplifiers can change with the level of RF output current. The same RF current flows in all of the power amplifiers. When the first power amplifier turns on, there is very little current flowing. When the second power amplifier turns on, the RF current doubles. As more and more amplifiers are turned on, the RF current increases to a relatively high level. When the RF current increases to a sufficiently high RF level, the on impedance of the transistor switches increases in magnitude. This will cause the voltage drop across these switching transistors to increase, thereby decreasing the RF voltage output from each power amplifier. Consequently, compensation is needed to offset the signal distortion resulting from operating such switching transistors at high RF current levels as well as from the variations in power supply discussed hereinabove.